This invention relates to a starting device for electric motors which is designed in particular to be fitted to compressor motors or the like, and consists of a PTC resistor in series with a relay.
The starting device in accordance with the invention, which is inserted between the line and the motor start winding, offers all the advantages deriving from the use of a PTC resistor without the limitations and drawbacks that this device would involve if used alone.
The contacts of the relay and the resistor are preferably housed in a single L-shaped box container so that a protection device can be inserted in the free space without increasing the overall size of the assembly. The device thus configured can be installed on the compressor merely by attaching it to the standardised connector fitted on the compressor. This system produces a more efficient, practical starting device which is easy and convenient to instal, and more functional.
In refrigerators, the unit constituted by the motor and compressor usually consists of a single, hermetically sealed block housed in a casing to which suitable connectors are fitted so that starting and protection devices can be connected to it.
These connectors are standardised, and usually comprise three male contacts to which the starting and protection device is connected. The starting and protection device comprises systems designed to supply the short-time pickup current needed to start the motor, together with thermal protection devices.
The present invention relates to a starting device of this kind. For a long time, a relay was used as the starting device; the relay closed for the time required to supply the motor windings with pickup current, and opened when the motor reached the required number of revolutions. More recently, relays have often been replaced by positive temperature coefficient (PTC) resistors, which present various advantages over relays. The PTC is a resistor whose resistance is added to that of the windings, so that the windings can be made with fewer coils, thus producing a saving of material and a slight reduction in size. A PTC resistor can consequently be advantageously used to replace the simple relay.
However, as the PTC resistor is always connected, it continues to absorb current, albeit to a minimal extent (a few watts per hour).
The aim of this invention is to offer a PTC motor starting device which does not present the drawback described above.
For this purpose, the present invention offers a starting device which comprises a relay and a PTC resistor in series.
When the motor has started, the relay opens and the PTC resistor is disconnected and stops absorbing current. The PTC resistor also protects the winding, because if the relay remains powered for any reason, for example if the contacts fail to open, the temperature of the PTC resistor increases, with the result that its resistance also increases and the winding is prevented from burning or being damaged.
It is thus possible to obtain the advantages deriving from the use of the PTC resistor without the associated drawbacks.